This invention relates to the area of coatings, and, in particular, to a surface-sealing composition for priming and topcoating substrates made from reaction injected molded (RIM) urethane, reinforced RIM urethane, and other materials which are sensitive to the organic solvents normally present in automotive primer and paint.
Soft fascia is a generic term used to describe flexible plastic automobile parts, such as bumper covers and front ends, which are usually painted to match the color of the vehicle body. A common soft fascia material is reaction injection molded urethane, although there are others including thermoplastics and EPDM (ethylene propylene diene monomer) polymers. Flexible plastic fenders which are glass reinforced reaction injection molded urethane are also being used by automobile manufacturers. In a collision, these components may sustain damage in the form of scratches, gouges, punctures, or tears. These components cannot be repaired by conventional techniques used for metal parts, e.g., welding or applying body patch or filler.
Reaction injection molded urethane and reinforced reaction injection molded urethane are microcellular foams which have highly cross-linked skins. When an automotive component made of RIM urethane or reinforced RIM urethane is repaired, the manufacturer-applied paint is removed by sanding. Sanding also removes the skin and exposes the underlying foam. The damaged area is filled with a repair material and sanded to a flat smooth surface. When primer and paint are applied, the repaired area appears to shrink, and it feels lower than the surrounding area. In actuality, the urethane foam substrate absorbs solvents from the primer and topcoat and swells around the patched area which was protected by the repair material. Although the absorbed solvents eventually evaporate, the paint film ceases to flow and level relatively quickly, with the result that the demarcation between the repair material and swollen substrate dries as a wrinkle in the paint.
In order to determine the severity of the solvent absorption problem, coupons of RIM urethane were allowed to soak twenty-four hours in a conventional lacquer thinner, DuPont 3661, manufactured by E. I. DuPont de Nemours and Co., in the major individual components of DuPont 3661, in certain selected organic solvents, and in water. The dimensions of the coupons were 3.0 cm.times.1.0 cm.times.0.32 cm. Both major surfaces of the coupons had skins. The edges of coupons did not have skins. The absorption of solvent by the urethane coupon was expressed as percent weight gain. The results are set forth in Table I:
TABLE I ______________________________________ Percent Weight Gain Solvent of RIM Urethane ______________________________________ DuPont 3661* 70.4 Acetone 72.4 Toluene 65.5 Butyl Acetate 62.0 "Cellosolve" Acetate 57.7 (ethylene glycol monoethyl ether acetate) Xylene 50.7 Methanol 39.7 Ethanol 27.8 1-Propanol 17.9 2-Propanol 12.2 1-Butanol 21.6 2-Butanol 20.2 tert-Butanol 5.1 1-Pentanol 11.3 2-Pentanol 11.8 3-Pentanol 7.7 2-Hexanol 9.9 Cyclohexanol 10.3 Hexane 11.5 Water 1.5 ______________________________________ *DuPont 3661 comprises 54% toluene, 14% "Cellosolve" acetate/butyl acetate, 12% isopropanol, 11% acetone, 6% xylene, 3% methanol.
It will be observed that aromatic hydrocarbons "oxygenated" solvents, i.e., ketones, esters, and ethers, and the DuPont lacquer thinner were absorbed to the extent of about 40 to about 70 percent of the weight of the urethane. Hexane, hexanols, pentanols, and isopropanol were absorbed at about the 8 to 13 percent level. Only water had little effect on the urethane samples. The coupon soaked in the lacquer thinner also experienced increases in length, width, and thickness of about 25 percent along each dimension. This dimensional increase represented a virtual doubling of volume. Because the eye can detect a scratch 0.1-0.2 micrometers deep, or 0.004% of the thickness of standard 0.32 cm RIM urethane, an increase in thickness of 25%, or about 800 micrometers, is readily noticeable.
The swelling of urethane around the repair material is evident when primer is applied. Where solvent is drawn away from the primer by the urethane substrate, the primer dries rapidly; however, it stays wet longer over the repair material. Expansion of the urethane foam can be observed at the boundary of the repair. When the primer has dried, the ridge can be sanded flat, but another application of primer or topcoat causes the ridge to develop again because solvents can permeate the primer. Several applications of primer with careful sanding of the ridge between coats can produce a perfect finish. However, this is a time-consuming process, resulting in an excessively thick layer of primer beneath the topcoat.